Three low molecular weight infectious RNAs (viroids), chrysanthemum stunt (ChSV), chrysanthemum chlorotic mottle (ChCMV-s), and potato spindle tuber (PSTV), will be investigated. The studies are designed to provide fundamental information on the mode of replication, and on the sequence homologies of viroid RNAs to one another, and to the DNA of their hosts. A symptomless strain of ChCMV-s (ChCMV-L) engenders no symptoms, but such an infection can protect the host from the severe disease caused by ChCMV-s. These viroid RNAs will be studied with respect to their: 1. Sequence homology, by fingerprint analysis of the labeled RNAs, and by hybridization with complimentary molecules. 2. Relationship of the viroid RNA to host and non-host DNA by molecular hybridization. 3. Possible genesis as induced by X-ray irradiation of potential host species. The investigations will provide fundamental information concerning the mode of replication of these very interesting and important small infectious RNAs, perhaps providing a model to guide future studies with vertebrate and plant systems. The finding of latency in some viroids and the possibility of protecting potential hosts is quite important because of the possibility of using them as immunizing agents against more deleterious isolates.